Binder for rechargeable battery with nonaqueous electrolyte and battery electrode depolarizing mix prepared using the same

ABSTRACT

To provide a binder for secondary battery using non-aqueous electrolyte which is soluble in usual organic solvents, does not swell in a non-aqueous electrolyte and besides enhances battery performance. As the binder, a copolymer comprising 50 to 80% by mole of vinylidene fluoride, 17 to 50% by mole of tetrafluoroethylene and less than 3% by mole of a monomer copolymerizable therewith is used.

TECHNICAL FIELD

The present invention relates to a binder for secondary battery usingnon-aqueous electrolyte and a battery electrode composition prepared byusing the binder.

BACKGROUND ART

Recently demand for electric and electronic apparatuses which aresmall-sized and portable, such as audio tape recorder, camera built-upvideo tape recorder, personal computer and portable phone has beenincreasing more and more. As a result of such an increased demand, highperformance secondary battery which is small-sized, light andrechargeable has come to be required. In addition to conventional leadstorage batteries and nickel-cadmium batteries, various new batteries ofnickel-metal hydride type and lithium ion type have been commercialized.Among them, nickel-metal hydride secondary batteries employing alkalineelectrolyte have problems remaining unsolved, that is to say, a voltageis low, energy density cannot be increased and a self-discharge islarge. On the other hand, lithium ion secondary batteries employingnon-aqueous electrolyte have merits such as a high voltage, high energydensity, small self-discharge and extra-light weight, and are expectedto be developed greatly in the future.

A point for enhancing energy density of the lithium ion secondarybatteries is a technique for producing an electrode thereof. In casewhere a negative electrode is produced by using a carbonaceous materialsuch as coke, carbon or the like as an active material for negativeelectrode, first the carbonaceous material is powdered, and thendispersed in a solvent together with a binder to prepare a negativeelectrode composition. After the composition is coated on a currentcollector for negative electrode, the solvent is removed by drying andthe coated current collector is rolled, thus giving the negativeelectrode. Hereinafter a carbonaceous material merely storing andreleasing a lithium ion is also called an active material. Similarly apositive electrode is produced, for example, by powdering alithium-containing oxide as an active material for positive electrode,dispersing the powder in a solvent together with a conductive agent andbinder to prepare a positive electrode composition, coating thecomposition on a current collector for positive electrode, removing thesolvent by drying and then rolling the coated current collector. As abinder for lithium ion secondary batteries, polyvinylidene fluoride hasbeen used widely. For example, JP-A-4-249859 discloses a technique forproducing electrode sheets, in which a lithium-containing oxide such asLiCoO₂ as an active material for positive electrode and graphite as aconductive agent are mixed with polyvinylidene fluoride to prepare apositive electrode composition, the obtained composition is dispersed inN-methylpyrrolidone to give a slurry, the slurry is applied onto analuminum foil current collector for positive electrode and then thecoated current collector is dried and compression-molded with a rollerpress, thus giving a positive electrode sheet, and separately in which acarbonaceous material as an active material for negative electrode ismixed with polyvinylidene fluoride to prepare a negative electrodecomposition, the obtained composition is dispersed inN-methylpyrrolidone to give a slurry, the slurry is coated on a copperfoil current collector for negative electrode and then the coatedcurrent collector is dried and compression-molded with a roller press,thus giving a negative electrode sheet. However, a solvent ofpolyvinylidene fluoride is limited to expensive specific organicsolvents having a high boiling point, such as N-methylpyrrolidone,dimethylformamide and dimethylacetamide. Therefore it takes a lot oftime to dry the solvent at the time of producing an electrode sheet andproduction cost increases. Also polyvinylidene fluoride is apt to swellin an organic solvent used for a non-aqueous electrolyte of lithium ionsecondary battery, such as propylene carbonate, ethylene carbonate,diethyl carbonate or a mixture thereof. For that reason, there ariseproblems that as charging and discharging are repeated, adhesion to ametal foil as a current collector becomes poor and as a result, increasein internal resistance of a battery occurs and battery performance islowered. Further an electrode sheet produced by using a polyvinylidenefluoride binder is poor in flexibility, and when the electrode sheet isfolded by 180 degrees for producing a square form battery or when woundfor producing a small cylindrical form battery, there easily occur aproblem that an electrode composition is separated from the electrodesheet, which results in decrease in yield. Also JP-A-4-95363 disclosesthe use, as a binder, of a material having rubber elasticity andcomprising mainly a fluorine-containing copolymer such as vinylidenefluoride-hexafluoropropylene copolymer or vinylidenefluoride-chlorotrifluoroethylene copolymer for the purpose to enhance abinding property against expansion and shrinkage of an active materialfor positive electrode at the time of charging and discharging of asecondary battery using a non-aqueous electrolyte. However since thosecopolymers have crystallinity lower than that of polyvinylidenefluoride, they are apt to swell against an organic solvent of anon-aqueous electrolyte as compared with polyvinylidene fluoride andelution thereof occurs depending on kind of a non-aqueous electrolyte.Thus they do not function as a binder. JP-B-8-4007 discloses the use ofsimilar binder such as fluorine-containing high molecular copolymermainly comprising vinylidene fluoride, tetrafluoroethylene andhexafluoropropylene instead of polyvinylidene fluoride. Claims of thatpatent publication discloses a copolymer comprising 0.3 to 0.9% by moleof vinylidene fluoride, 0.03 to 0.5% by mole of hexafluoropropylene and0 to 0.5% by mole of tetrafluoroethylene, in which a total % by mole ofthose three monomers is from 0.80 to 1. In that patent publication, too,it is pointed out that since polyvinylidene fluoride is soluble only inthe above-mentioned specific solvents such as N-methylpyrrolidone,dimethylacetamide, dimethylformamide and methyl sulfoxide which have ahigh polarity and a high boiling point and some of which are toxic, whenan electrode is produced by coating an active material by using thementioned solvent and then molding, there are problems, from theviewpoint of production process, that it takes too long period of timeto dry the solvent having a high boiling point and due to toxicity ofthe solvent, sealing equipment and exhaust equipment are required. Inthat patent publication, in order to solve the above-mentioned problems,the mentioned copolymer which dissolves in usual organic solventscosting low and having a low boiling point, for example, ketone solventssuch as methyl ethyl ketone and methyl isobutyl ketone, ester solventssuch as ethyl acetate and butyl acetate, ether solvents such as dioxaneand tetrahydrofuran and a mixture thereof is used for a binder. However,since a degree of swelling of that copolymer is basically large againstan organic solvent of non-aqueous electrolyte like the above-mentionedvinylidene fluoride-hexafluoropropylene copolymer and vinylidenefluoride-chlorotrifluoroethylene copolymer, during a long term use of abattery, peeling of a battery electrode composition from a currentcollector and releasing of an active material occur, which arises aproblem with lowering of battery performance.

Further JP-A-7-147156 describes that by using an electrode produced byadhering a composite layer of an insoluble non-melting substrate havinga polyacene structure and a specific binder to a metal foil, batteryperformance is enhanced and, that the binder is a fluorine-containingpolymer having a fluorine atom/carbon atom ratio of less than 1.5 andnot less than 0.75. However a polymer disclosed in that patentpublication is only polyvinylidene fluoride. For example, bothethylene-tetrafluoroethylene copolymer and propylene-tetrafluoroethylenecopolymer disclosed therein are insoluble in an organic solvent, andseem not suitable as a binder and not practical from the description inthe publication that in order to obtain a uniform electrode, it ispreferable that a fluorine-containing polymer is completely dissolved.

An object of the present invention is to provide a binder for secondarybattery using non-aqueous electrolyte which has flexibility as comparedwith polyvinylidene fluoride, is soluble in not only conventionalsolvents such as N-methylpyrolidone, dimethylacetamide anddimethylformamide but also an organic solvent having a low boiling pointsuch as acetone or methyl ethyl ketone and, as compared with theabove-mentioned fluorine-containing copolymer, is less swelling againstorganic solvents of non-aqueous electrolyte such as propylene carbonate,ethylene carbonate, diethyl carbonate, diethoxyethane and a mixturethereof, and to provide a battery electrode composition which isprepared by using the binder.

DISCLOSURE OF THE INVENTION

The present inventors have found, as a result of their study, that theuse of a copolymer mainly comprising vinylidene fluoride andtetrafluoroethylene as a binder for secondary battery using non-aqueouselectrolyte makes it possible to simplify production process and reduceproduction cost, and that since a degree of swelling of the copolymeragainst the non-aqueous electrolyte lowers, battery performance can beenhanced.

Namely, the present invention relates to the binder for secondarybattery using non-aqueous electrolyte, in which the secondary batterycomprises a positive electrode produced by adhering a positive electrodecomposition comprising an active material for positive electrode,conductive agent and binder to a current collector for positiveelectrode and/or a negative electrode produced by adhering a negativeelectrode composition comprising an active material for negativeelectrode and binder to a current collector for negative electrode and anon-aqueous electrolyte, and the binder comprises a copolymer of twomonomers comprising 50 to 80% by mole of vinylidene fluoride and 20 to50% by mole of tetrafluoroethylene.

Also the present invention relates to the binder for secondary batteryusing non-aqueous electrolyte, which comprises a copolymer of three ormore monomers of 50 to 80% by mole of vinylidene fluoride, not less than17% by mole and less than 50% by mole of tetrafluoroethylene and lessthan 3% by mole of a monomer copolymerizable therewith.

It is preferable that a molecular weight of the above-mentionedcopolymers is larger from the viewpoint of enhancing cyclecharacteristics. For example, a preferred number average molecularweight of copolymer comprising three or more monomers is from 150,000 to500,000.

The binder of the present invention is useful particularly in case wherea lithium-containing oxide is used as an active material for positiveelectrode.

Also the present invention relates to the battery electrode compositionwhich contains the binder mentioned above.

BEST MODE FOR CARRYING OUT THE INVENTION

The binder of the present invention is characterized by comprising avinylidene fluoride-tetrafluoroethylene copolymer, in which a proportionof vinylidene fluoride is from 50 to 80% by mole, preferably from 60 to80% by mole.

Examples of the binder are copolymer of two monomers comprising 50 to80% by mole, preferably 60 to 80% by mole of vinylidene fluoride and 20to 50% by mole, preferably 20 to 40% by mole of tetrafluoroethylene andcopolymer comprising three or more monomers of 50 to 80% by mole,preferably 60 to 80% by mole of vinylidene fluoride, not less than 17%by mole and less than 50% by mole, preferably not less than 17% by moleand less than 40% by mole of tetrafluoroethylene and less than 3% bymole, preferably less than 2.8% by mole, further preferably less than2.5% by mole of monomer copolymerizable therewith.

The vinylidene fluoride copolymer used in the present invention can beprepared by known polymerization methods. Among them, preferable isradical polymerization method. Namely as far as the polymerization isproceeded radically, there is no limit in polymerization means. Thepolymerization is started with, for example, organic or inorganicradical initiator, heat, light, radioactive ray or the like. Alsosolution polymerization, bulk polymerization, suspension polymerization,emulsion polymerization, or the like can be employed.

In case where vinylidene fluoride is less than 50% by mole, it isdifficult to dissolve the copolymer throughout an organic solvent. Onthe other hand, if more than 80% by mole, a degree of swelling of thecopolymer against electrolytes such as propylene carbonate, ethylenecarbonate and diethyl carbonate is increased. As a result, if it is usedas the binder in the larger amount, when a battery is used for a longperiod of time or when used continuously at high temperature, peeling ofthe battery electrode composition from the current collector andreleasing of the active material occur, which results in lowering ofbattery performance. Further if its amount is more than 80% by mole,dissolution to usual organic solvents having a low boiling point becomespoor, and the specific organic solvent having high boiling point such asN-methylpyrrolidone or dimethylformamide is necessarily used. Therefore,it takes a longer period of time for drying the solvent when producingbatteries, and production efficiency cannot be enhanced. Also since theobtained copolymer becomes hard and lacks in flexibility, there is alimit in small circular winding or folding of the electrode sheet anddifficulty in enhancing battery performance arises.

A preferred molecular weight of the copolymer comprising two monomers orthree or more monomers of the present invention is from 10,000 to500,000 according to a number average molecular weight measured by GPC(gel permeation chromatography) based on polystyrene conversion. Amolecular weight of less than 10,000 is too low to form a film, and whenmore than 500,000, there is a tendency that pseudoplasticity of thebattery electrode composition increases greatly and it becomes difficultto apply the composition to the current collector for electrode. Inorder to enhance cycle characteristics, it is preferable that themolecular weight is relatively larger. From that point of view, in caseof the copolymer comprising, for example, three or more monomers, apreferred molecular weight is from 150,000 to 500,000.

For adhesion of the binder to the current collector, the copolymer oftetrafluoroethylene and vinylidene fluoride suffices. The adhesion canbe further enhanced by copolymerizing the monomer copolymerizable withthe copolymer in an amount of not impairing excellent swelling propertyof the copolymer in the non-aqueous electrolyte. The copolymerizablemonomer may be added in an amount of less than 3% by mole. If it isadded in an amount of not less than 3% by mole, in general there is atendency that crystallinity of the copolymer of vinylidene fluoride andtetrafluoroethylene is lowered significantly, and as a result, swellingproperty in non-aqueous electrolyte becomes poor. Examples of themonomer copolymerizable with vinylidene fluoride and tetrafluoroethyleneare unsaturated dibasic acid monoesters described in JP-A-6-172452, forexample, monomethyl maleate, monomethyl citraconate, monoethylcitraconate, vinylene carbonate and the like; compounds described inJP-A-7-201316 and having hydrophilic polar group such as —SO₃M, —OSO₃M,—COOM, —OPO₃M (M represents an alkali metal), or amine type polar groupsuch as —NHR¹ or —NR²R³ (R¹, R² and R³ are alkyl groups), for example,CH₂═CH—CH₂—Y, CH₂═C(CH₃)—CH₂—Y, CH₂═CH—CH₂—O—CO—CH(CH₂COOR⁴)—Y,CH₂═CH—CH₂—O—CH₂—CH(OH)—CH₂—Y, CH₂═C(CH₃)—CO—O—CH₂—CH₂—CH₂—Y,CH₂═CH—CO—O—CH₂—CH₂—Y and CH₂═CH—CO—NH—C(CH₃)₂—CH₂—Y (Y represents ahydrophilic polar group and R⁴ represents an alkyl group); maleic acid;maleic anhydride; and the like. Further as the copolymerizable monomer,there can be also used hydrated allyl ether monomers such asCH₂═CH—CH₂—O—(CH₂)_(n)—OH (3≦n≦8),

CH₂═CH—CH₂—O—(CH₂—CH₂—O)_(n)—H (1≦n≦14) andCH₂═CH—CH₂—O—(CH₂—CH(CH₃)—O)_(n)—H (1≦n≦14), and carboxylated allylether and ester monomers and/or allyl ether and ester monomerssubstituted by —(CF₂)_(n)—CF₃ (3≦n≦8), for example,CH₂═CH—CH₂—O—CO—C₂H₄—COOH, CH₂═CH—CH₂—O—CO—C₅H₁₀—COOH,CH₂═CH—CH₂—O—C₂H₄—(CF₂)_(n)CF₃, CH₂═CH—CH₂—CO—O—C₂H₄—(CF₂)_(n)CF₃,CH₂═C(CH₃)—CO—O—CH₂—CF₃ and the like. By the way, studies having beenmade so far made it possible to expect that even by using a compoundother than the above-mentioned compound having a polar group, adhesionto the current collector made of aluminum or copper foil can be enhancedby lowering slightly crystallinity of the copolymer of vinylidenefluoride and tetrafluoroethylene and thus endowing the copolymermaterial with flexibility. Thereby there can be used, for example,unsaturated hydrocarbon monomers (CH₂═CHR, in which R is hydrogen, analkyl group or a halogen such as C1) such as ethylene and propylene;fluorine-containing monomers such as chlorotrifluoroethylene,hexafluoropropylene and hexafluoroisobutene; CF₂═CF—O—C_(n)F_(2n+1) (nis an integer of 1 or more); CH₂═CF—C_(n)F_(2n+1) (n is an integer of 1or more); CH₂═CF—(CF₂CF₂)_(n)H (n is an integer of 1 or more); andCF₂═CF—O—(CF₂CF(CF₃)O)_(m)—C_(n)F_(2n+1) (each of m and n is an integerof 1 or more). In addition to the above-mentioned compounds, there canbe used at least one of fluorine-containing ethylenically unsaturatedmonomers having a functional group and represented by the formula (1):

wherein Y is —CH₂OH, —COOH, a carboxylic salt group, a carboxylic estergroup or epoxy, X and X¹ are the same or different and each is hydrogenatom or fluorine atom, R_(f) is a divalent fluorine-containing alkylenegroup having 1 to 40 carbon atoms or a divalent fluorine-containingalkylene group having ether bond and 1 to 40 carbon atoms. Bycopolymerizing one or two or more of those monomers, adhesion to thecurrent collector is further enhanced, and even if charging anddischarging are repeated, the active material of electrode is not peeledoff from the current collector and good charging and discharging cyclecharacteristics can be obtained.

The copolymer mainly comprising vinylidene fluoride andtetrafluoroethylene which is used as the binder for battery of thepresent invention (hereinafter may be referred to as “vinylidenefluoride-tetrafluoroethylene copolymer”) has a lower degree of swellingin the above-mentioned organic electrolyte despite that it is solublenot only in nitrogen-containing organic solvents such asN-methylpyrrolidone, dimethylformamide and dimethylacetamide which aresolvents of polyvinylidene fluoride but also usual organic solventshaving a low boiling point which are used well in general. Therefore byusing such a copolymer, the battery electrode composition and electrodesheet which have flexibility can be obtained.

Examples of the usual organic solvents having a low boiling point are,for instance, ketone solvents such as acetone, methyl ethyl ketone,cyclohexanone and methyl isobutyl ketone; ester solvents such as ethylacetate and butyl acetate; ether solvents such as tetrahydrofuran anddioxane; and a mixture thereof.

In case where the above-mentioned copolymer is used as a binder, it isusual to disperse and mix electrode materials such as an active materialfor electrode in a solution in which the copolymer is dissolved. Inaddition, for example, the composition may be prepared by previouslymixing the copolymer powder and a powder of the active material forelectrode and then adding the organic solvent thereto. Also theelectrode sheet can be produced by heating and melting powders of thecopolymer and active material for electrode, extruding with an extrusionmolding machine to prepare the composition in the form of thin film andthen laminating the film to the current collector coated with theconductive agent and the above-mentioned usual organic solvent. Furtherthe solution of the copolymer may be applied to a preformed activematerial for electrode. As mentioned above, a method for applying thecopolymer as the binder is not limited.

As one of methods for enhancing adhesion of the composition containingthe binder comprising the copolymer with the current collector, there isa method of drying the composition coating on the current collector at atemperature of not less than the melting point of the copolymercontained in the binder. However in case of polyvinylidene fluoridewhich is a conventional binder, JP-A-4-249859 describes that when thecomposition coating is dried at a relatively high temperature (not lessthan the temperature of 170° to 180° C. In general, a melting point ofpolyvinylidene fluoride is around 175° C.), deterioration ofpolyvinylidene fluoride occurs and as a result, a factor for maintaininga battery capacity of charge and discharge cycle characteristics islowered. In case of the copolymer used in the present invention, such aproblem does not occur by heating and drying treatments, and desiredenhancement of adhesion is observed and battery performance is stable.

In the binder of the present invention, in order to further enhanceadhesion, a resin such as polymethacrylate, polymethyl methacrylate,polyacrylonitrile, polyimide, polyamide, polyamideimide or polycarbonatemay be contained in the above-mentioned copolymer comprising twomonomers or three or more monomers. It is preferable that a content ofthe resin in the binder is not more than about 20% by volume.

Secondary battery for non-aqueous electrolyte to which the binder of thepresent invention is applied comprises a positive electrode produced byadhering the positive electrode composition comprising the activematerial for positive electrode, conductive agent and binder to thecurrent collector for positive electrode; a negative electrode producedby adhering the negative electrode composition comprising the activematerial for negative electrode and binder to the current collector fornegative electrode; and the non-aqueous electrolyte.

The present invention also relates to the battery electrode compositionwhich comprises the above-mentioned binder and other electrodematerials. As the other electrode materials, there are an activematerial for positive electrode, active material for negative electrodeand conductive agent which are described below.

Examples of the active material for positive electrode which can be usedin the present invention are transition metal oxides such as manganesedioxide and vanadium pentoxide; transition metal chalcogenides such asiron sulfide and titanium sulfide; oxide mixtures containing lithium;and the like. Particularly from the viewpoint that high voltage and highenergy density can be obtained and charge and discharge cyclecharacteristics are excellent, preferable are lithium-containing oxidesrepresented by the formula: Li_(X)A_(1—Y)M_(Y)O₂ (A is at least onetransition metal element selected from the group consisting of Mn, Coand Ni, M is at least one element selected from the group consisting ofB, Mg, Ca, Sr, Ba, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Al, In, Nb, Mo, W, Yand Rh, 0.05≦X≦1.1, 0≦Y≦0.5). Examples of desirable oxide are, forinstance, lithium cobalt dioxide (LiCoO₂), lithium nickel dioxide(LiNiO₂), lithium manganese tetraoxide (LiMn₂O₄) and the like.

As the active material for negative electrode, there can be usedcarbonaceous materials being capable of doping/un-doping lithium and thelike. Examples of the preferred material are, for instance, electricallyconductive polymers such as polyacene and polypyrole, cokes, polymercarbon, carbon fiber, and besides, from the viewpoint of a large energydensity per unit volume, pyrolitic carbon, cokes (petroleum cokes, pitchcokes, coal cokes and the like), carbon black (acetylene black and thelike), vitreous carbon, sintered organic high molecular material(article produced by sintering organic high molecular material at atemperature of not less than 500° C. in inert gas stream or in vacuum),and the like.

Examples of the conductive agent are, for instance, carbonaceousmaterials, i.e., carbon blacks such as acetylene black and Ketjen black,graphites and the like.

The binder of the present invention is used as the binder in thepositive electrode composition and/or the binder in the negativeelectrode composition, and its amount is from 0.1 to 20% by weight,preferably from 1 to 10% by weight based on the electrode composition.The residual amount is that of the above-mentioned electrode materials.

The current collector for positive electrode, to which the electrodecomposition is laminated is, for example, an aluminum foil or the like.The current collector for negative electrode is a copper foil or thelike.

The non-aqueous electrolyte is not particularly limited. As the organicsolvent, there can be used one or two or more of known solvents such aspropylene carbonate, ethylene carbonate, butylene carbonate,γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, dimethylcarbonate and diethyl carbonate. Also there can be used any of knownelectrolytes such as LiClO₄, LiAsF₆, LiPF₆, LiBF₄, LiCl, LiBr, CH₃SO₃Li,CF₃SO₃Li and cesium carbonate. The battery electrode composition of thepresent invention may be used in combination of an acrylic resin such aspolymethacrylate or polymethyl methacrylate, a polyimide, polyamide orpolyamideimide resin or the like to enhance adhesion to the currentcollector.

The binder of the present invention is useful not only for lithium ionsecondary battery using the above-mentioned liquid electrolyte, as thebinder for secondary battery using non-aqueous electrolyte but also forlithium secondary battery using a polymer electrolyte, a polymerelectrolyte (so-called high molecular gel electrolyte) being holding anelectrolytic solution or electrolyte and taking a role of a separator.

The present invention is then explained based on examples, but is notlimited to them.

EXAMPLES 1 TO 9

As shown in Table 1, a copolymer of vinylidene fluoride andtetrafluoroethylene and a terpolymer prepared by copolymerizing thosemonomers with other copolymerizable monomer for enhancing adhesiveproperty were prepared by usual method. A proportion thereof, andmolecular weight and solubility of the copolymer in an organic solventare shown in Table 1. The results of Comparative Examples 1 to 4 whereincopolymers and polyvinylidene fluoride (VP825 available from DAIKININDUSTRIES, LTD.) used are also shown in Table 1.

The solubility in an organic solvent was determined by using 10% byweight of copolymer shown in Table 1 per each solvent at a temperatureof from room temperature to 50° C. In the table, ◯ represents beingsoluble, and x represents being insoluble.

TABLE 1 Proportion of vinylidene Proportion of Other copolymerizablemonomer Number average fluoride tetrafluoroethylene Proportion molecularweight* (% by mole) (% by mole) Kind (% by mole) (Mn × 10⁴) Ex. 1 65 35— 0 18.4 Ex. 2 70 30 — 0 20.0 Ex. 3 75 25 — 0 19.0 Ex. 4 63.6 36.1Monomethyl maleate 0.3 12.8 Ex. 5 61.5 37.0 CF₂═CF(CF₃) 1.5 14.0 Ex. 661.7 37.3 CF₂═CF—O—C₃F₇ 1.0 15.0 Ex. 7 61.4 36.2 CF₂═CF(CF₃) 2.4 13.0Ex. 8 61.5 35.7 CF₂═CF(CF₃) 2.8 10.0 Ex. 9 65 32.5 CF₂═CF(CF₃) 2.5 25Com. Ex. 1 45 (estimated) 55 (estimated) — 0 unknown Com. Ex. 2 90 10 —0 20.0 Com. Ex. 3 100 0 — 0 12.0 Com. Ex. 4 61.9 35.0 CF₂═CF(CF₃) 3.114.0 Solubility in organic solvent NMP THF MEK Acetone Ex. 1 ∘ ∘ ∘ ∘ Ex.2 ∘ ∘ ∘ ∘ Ex. 3 ∘ ∘ ∘ ∘ Ex. 4 ∘ ∘ ∘ ∘ Ex. 5 ∘ ∘ ∘ ∘ Ex. 6 ∘ ∘ ∘ ∘ Ex. 7∘ ∘ ∘ ∘ Ex. 8 ∘ ∘ ∘ ∘ Ex. 9 ∘ ∘ ∘ ∘ Com. Ex. 1 x x x x Com. Ex. 2 ∘ x xx Com. Ex. 3 ∘ x x x Com. Ex. 4 ∘ ∘ ∘ ∘ (NMP: N-Methylpyrrolidone, THF:Tetrahydrofuran, MEK: Methyl ethyl ketone) *Number average molecularweight based on polystyrene

EXAMPLE 10

A metal die having a diameter of 12 cm was charged with 10 g ofcopolymer powder of Examples 1 to 9, followed by pressing at 230° C. ata gauge pressure of 15 kg/cm² with a 50-ton press to give a sheet ofabout 0.5 mm thick. Polyvinylidene fluoride of Comparative Example 3 waspress-molded in the same manner as above to give a sheet. Those sheetswere cut into strips having a width of 0.5 cm and a length of 3 cm, anda dynamic viscoelasticity was measured (with a meter available fromRheometrics Co., Ltd., Frequency: 3.5 Hz, 25° C.). The results are shownin Table 2.

TABLE 2 Dynamic viscoelasticity Copolymer (x 10⁹ dyn/cm²) Ex. 1 6.0 Ex.2 8.0 Ex. 3 10.0 Ex. 4 6.4 Ex. 5 5.2 Ex. 6 3.8 Ex. 7 3.8 Ex. 8 3.5 Ex. 92.3 Com. Ex. 3 15.0

EXAMPLE 11

The sheets produced in Example 10 by molding copolymers of Examples 1 to9 were cut into strips having a width of 1 cm and a length of 5 cm, andvolumes thereof were measured with a specific gravity meter (DENSI METERavailable from Toyo Seiki Co., Ltd.). Then the strips were dipped in apropylene carbonate and ethylene carbonate mixture (volume ratio 1:1)heated to 85° C. and an ethylene carbonate and diethyl carbonate mixture(volume ratio 1:1) heated to 85° C., respectively. A volume change ratio(a volume ratio increased by dipping) after 72-hour dipping wasmeasured. The same measurements were made for comparison purpose withrespect to strips having the same size and produced from sheets obtainedby molding polyvinylidene fluoride and copolymer powder of ComparativeExamples 3 to 4 in the same manner as in Example 10 and also withrespect to strips having the same size and produced from sheets obtainedby molding a fluorine-containing rubber of a terpolymer of vinylidenefluoride (VdF)-tetrafluoroethylene (TFE)-hexafluoropropylene (HFP)(vinylidene fluoride/tetrafluoroethylene/hexafluoropropylene=60/20/20%by mole) in the same manner as in Example 10. The results are shown inTable 3.

TABLE 3 Volume change ratio in electrolyte (%) Propylene carbonate-Ethylene carbonate- ethylene carbonate diethyl carbonate Electrolytemixture mixture Ex. 1 10 18 Ex. 2 28 36 Ex. 3 32 40 Ex. 4 20 23 Ex. 5 2324 Ex. 6 21 30 Ex. 7 27 33 Ex. 8 39 45 Ex. 9 40 45 Com. Ex. 3 40 46 Com.Ex. 4 50 Large swelling (unmeasurable) VdF-TFE-HFP 75 Dissolvedterpolymer

EXAMPLE 12 Production of Negative Electrode

Sixty parts by weight of carbon black as an active material for negativeelectrode, 5 parts by weight each of polymer of Example 1 (copolymer),Examples 4 to 6 and 9 (terpolymer) or polyvinylidene fluoride ofComparative Example 3 as a binder and 35 parts by weight ofN-methylpyrrolidone, methyl ethyl ketone or tetrahydrofuran as a solventwere mixed for 10 hours by using a ball mill to give a negativeelectrode composition. The composition was coated on both surfaces ofcopper foil of 10 μm thick which would become a current collector fornegative electrode, so that a thickness after drying became 100 μm.Finally the coated copper foil was dried at 120° C. and then rolled togive a negative electrode strip.

Production of Positive Electrode

Sixty parts by weight of LiCoO₂ as an active material for positiveelectrode, 5 parts by weight of acetylene black as a conductive agent, 5parts by weight each of polymer of Example 1 (copolymer), Examples 4 to6 and 9 (terpolymer) or polyvinylidene fluoride of Comparative Example 3as a binder and 30 parts by weight of N-methylpyrrolidone, methyl ethylketone or tetrahydrofuran as a solvent were mixed for 10 hours by usinga ball mill to give a positive electrode composition. The compositionwas coated on both surfaces of aluminum foil of 20 μm thick which wouldbecome a current collector for positive electrode, so that a thicknessafter drying became 100 μm. Finally the coated aluminum foil was driedat 120° C. and then rolled to give a positive electrode strip.

Production of Battery

A battery was produced according to the method described inJP-A-7-201316 by using the negative electrode strip and the positiveelectrode strip which were produced in the manner mentioned above.

Namely the above-mentioned positive electrode strip and negativeelectrode strip were laminated through a 25 μm thick polypropylene filmwhich was a separator, and the laminated article was wound plural timesto give a spiral electrode having an outside diameter of 18 mm. Thespiral electrode was put in a battery container made of nickel-platediron, and insulating sheets were provided on the top and bottom of thespiral electrode. An aluminum lead wire for positive electrode was ledout from the current collector for positive electrode and welded to abattery cap, and a nickel lead wire for negative electrode was led outfrom the current collector for negative electrode and welded to thebattery container.

To the battery container in which the spiral electrode was put wasfilled a non-aqueous electrolyte prepared by dissolving LiPF₆ at aconcentration of 1 mole/liter in a solvent comprising ethylene carbonateand diethyl carbonate at a volume ratio of 1:1. Then a safety valvehaving a current shut-off mechanism and a battery cap were caulked andfixed to the battery container via an insulating sealing gasket of whichsurface was coated with asphalt. Thus cylindrical secondary batterieshaving a diameter of 18 mm and a height of 65 mm and using a non-aqueouselectrolyte were produced (Those batteries are represented by A to P inTable 4).

With respect to the secondary batteries so-produced by using anon-aqueous electrolyte, charging was carried out for 2.5 hours at roomtemperature under the conditions of 4.2 V of maximum charge voltage and1 A of charge current and then discharging was carried out at 6.2 Ω of aconstant resistance. Thus charge and discharge cycles were repeated anda change in discharge capacity was measured to determine the number ofcycles, in which discharge capacity lowers to 50% of initial capacity(number of cycles at 50% discharge capacity). The results are shown inTable 4.

TABLE 4 Number of cycles at 50% discharge Battery No. Kind of binderKind of solvent capacity A Example 1 N-methylpyrrolidone 607 B Same asabove Methyl ethyl ketone 625 C Same as above Tetrahydrofuran 600 DExample 4 N-methylpyrrolidone 610 E Same as above Methyl ethyl ketone595 F Same as above Tetrahydrofuran 622 G Example 5 N-methylpyrrolidone606 H Same as above Methyl ethyl ketone 680 I Same as aboveTetrahydrofuran 692 J Example 6 N-methylpyrrolidone 694 K Same as aboveMethyl ethyl ketone 601 L Same as above Tetrahydrofuran 611 M Example 9N-methylpyrrolidone 750 N Com. Ex. 3 N-methylpyrrolidone 510 O Same asabove Methyl ethyl ketone * P Same as above Tetrahydrofuran **Polyvinylidene fluoride of Comparative Example 3 was not dissolved insolvents other than N-methylpyrrolidone, and a coating composition couldnot be prepared.

As is clear from the results of Table 4, as compared with Batteries N toP produced by using polyvinylidene fluoride as the binder, the number ofcycles at 50% capacity of Batteries A to M produced by using thevinylidene fluoride-tetrafluoroethylene copolymer was large, and goodcharge and discharge cycle characteristics were exhibited. It can beconsidered that the reason why the cycle characteristic of Battery M wasparticularly good is that since the copolymer having a larger numberaverage molecular weight as compared with other copolymers was used asthe binder, adhesion of the electrode composition with the aluminum foilor copper foil which was the current collector was further enhanced.

From the above-mentioned results, it is seen that the use of vinylidenefluoride-tetrafluoroethylene copolymer, particularly copolymer having arelatively large number average molecular weight as the binder is usefulfor enhancing charge and discharge cycle characteristics of batteries.

EXAMPLE 13 Production of Negative Electrode

The same procedures as in Example 12 were repeated to make a negativeelectrode strip except that N-methylpyrrolidone was used as the solventfor the binder and the drying temperature of the composition was changedto 190° C.

Production of Positive Electrode

The same procedures as in Example 12 were repeated to make a positiveelectrode strip except that N-methylpyrrolidone was used as the solventfor the binder and the drying temperature of the composition was changedto 190° C.

Production of Battery

Secondary batteries using a non-aqueous electrolyte were produced in thesame manner as in Example 12 by using the thus produced negativeelectrode strip and positive electrode strip (Those batteries arerepresented by Q to U in Table 5), and the tests (number of cycles at50% discharge capacity) were carried out in the same manner as inExample 12. The results are shown in Table 5.

TABLE 5 Number of cycles at 50% Battery No. Kind of binder dischargecapacity Q Example 1 634 R Example 4 606 S Example 5 595 T Example 6 612U Com. Ex. 3 452

As is clear from the results of Table 5, as compared with Battery Uproduced by using polyvinylidene fluoride as the binder, the number ofcycles at 50% discharge capacity of Batteries Q to T produced by usingthe vinylidene fluoride-tetrafluoroethylene copolymer was large, andgood charge and discharge cycle characteristics were exhibited.

From the above-mentioned results, it is seen that the use of thevinylidene fluoride-tetrafluoroethylene copolymer as the binder isuseful for enhancing charge and discharge cycle characteristics ofbatteries.

INDUSTRIAL APPLICABILITY

According to the present invention, there can be obtained the binder forsecondary battery using non-aqueous electrolyte which has flexibility ascompared with conventional polyvinylidene fluoride, is soluble in notonly conventional solvents such as N-methylpyrrolidone,dimethylacetamide and dimethylformamide but also usual organic solventssuch as acetone and methyl ethyl ketone having a low boiling point, andis less swelling against an organic solvent of non-aqueous electrolyteas compared with conventional fluorine-containing copolymers orterpolymers, and the battery electrode composition can be obtained. As aresult, in view of battery production, simplification of productionfacilities and increase in yield can reduce production cost, and in viewof battery performance, the battery having large number of cycles at 50%discharge capacity and exhibiting good charge and discharge cyclecharacteristics can be provided.

What is claimed is:
 1. A binder for secondary battery produced by usingnon-aqueous electrolyte, in which the secondary battery comprises apositive electrode produced by adhering a positive electrode compositioncomprising an active material for positive electrode, conductive agentand binder to a current collector for positive electrode and/or anegative electrode produced by adhering a negative electrode compositioncomprising an active material for negative electrode and binder to acurrent collector for negative electrode and a non-aqueous electrolyte;the binder comprising a copolymer consisting essentially of 50 to 80% bymole of vinylidene fluoride and 20 to 50% by mole oftetrafluoroethylene.
 2. The binder for secondary battery produced byusing non-aqueous electrolyte of claim 1, in which the binder containedin said positive electrode composition and/or negative electrodecomposition comprises a copolymer consisting essentially of 50 to 80% bymole of vinylidene fluoride, not less than 17% by mole and less than 50%by mole of tetrafluoroethylene and less than 3% by mole of one or moremonomers copolymerizable therewith.
 3. The binder for secondary batteryproduced by using non-aqueous electrolyte of claim 2, wherein a numberaverage molecular weight of said copolymer is from 150,000 to 500,000.4. The binder for secondary battery produced by using non-aqueouselectrolyte of claim 1, wherein the active material for positiveelectrode is a lithium-containing oxide.
 5. A battery electrodecomposition which contains the binder for secondary battery produced byusing non-aqueous electrolyte of claim
 1. 6. A secondary batteryproduced by using non-aqueous electrolyte, which contains the batteryelectrode composition of claim
 5. 7. The binder for secondary batteryproduced by using non-aqueous electrolyte of claim 2, wherein the activematerial for positive electrode is a lithium-containing oxide.
 8. Thebinder for secondary battery produced by using non-aqueous electrolyteof claim 3, wherein the active material for positive electrode is alithium-containing oxide.
 9. A battery electrode composition whichcontains the binder for secondary battery produced by using non-aqueouselectrolyte of claim
 2. 10. A battery electrode composition whichcontains the binder for secondary battery produced by using non-aqueouselectrolyte of claim
 3. 11. A battery electrode composition whichcontains the binder for secondary battery produced by using non-aqueouselectrolyte of claim
 7. 12. A battery electrode composition whichcontains the binder for secondary battery produced by using non-aqueouselectrolyte of claim
 8. 13. A secondary battery produced by usingnon-aqueous electrolyte, which contains the battery electrodecomposition of claim
 9. 14. A secondary battery produced by usingnon-aqueous electrolyte, which contains the battery electrodecomposition of claim
 10. 15. A secondary battery produced by usingnon-aqueous electrolyte, which contains the battery electrodecomposition of claim
 11. 16. A secondary battery produced by usingnon-aqueous electrolyte, which contains the battery electrodecomposition of claim 12.